M920 CAT7155 Transmission Help needed

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GAR16

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Perry, OK
We are having problems with our CAT 7155 transmission and are thinking about swapping it out. What auto transmission will fit?
 

GAR16

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Perry, OK
Here's our problem...The truck, M920, only has 200 miles on it. With the air shift CAT 7155 trans we have had consistant problems getting it to go into reverse. Now we are also having problems getting it to shift into the forward gears. It has always needed at least 120 PSI to go into reverse but that does not always work. We have already pured in 4 oz of Marvel air tool oil. Any new suggestions?aua
 

R Racing

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St. Leonard, MD
Is it not going into gear? or is it going into gear and not moving ? Also do you hear air hissing from either the shifter or the vents ? I have to put in about 8 to 10 oz of oil before using my M920 . You may have a damaged O ring. Cat still supports the trans and at least in MD still works on them.
 

R Racing

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St. Leonard, MD
2 places. 1st on the right side of the shifter ( when sitting in drivers seat) is a umbilical cord. it contains 13 air lines in what looks to be a cable. Remove the 3 each 1/2 " headed bolts ( make sure your air system is drained and has no pressure) pull the head away from the shifter and rotate the side with the air holes facing up. Pour oil in all the holes as much as will go in. ( give it a min to let the oil weep into the hole and repeat ) Now bolt the umbilical cord back to the shifter. Next go to the drives air tank located under the drivers door Should be a Red line if it isnt painted green . Also with no air in the system remove the air hose from the rear of the tank and fill it up with airtool oil . It helps if while filling you hold the hose up higher than the connection to force the oil over the hump that the line goes over. Reconnect the line ( I removed that 90 degree fitting from the tank and drilled and taped a 1/4 pipe hole so i dont have to remove the air line, Mine was a bugger to get back on. Now I just remove the plug and pour the oil in the hole and reinstall the plug wish I could upload pictures of it. ) And start the truck and let it build pressure to 120 ( I adjusted my air governer to 135 psi to help the shifting process, If you need help on how to adjust it let me know ) and the push in the button and shift up to 6th and back thru R2 until the oil does its trick. most of the time its the rear clutch not getting enough air due to a bad or dry O ring.
 
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GAR16

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Perry, OK
R Racing, thanks for the info...will do tomorrow. When you say to shift up to 6 then down to R2, meaning every gear until the oil does it's trick? How do we adjust our air governer? Also, how often do you have to add oil to your M920?
 

R Racing

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2,757
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Location
St. Leonard, MD
I add Oil to the tank line every time I use the truck ( I only use the truck about once every 2 weeks ) I do the Umbilical cord about every 3rd or 4th use unless it just doesn't want to work . Then I do it. If I use the truck more often lets say 3 days in a row I don't have any issues. But if it sits more than a week I just pour in the airtool oil . Shift thru the gears and in a few min I'm good to go.

Ok now the Governer . Its located on top of the air compressor and has several air lines coming out of it. to the front of the truck on the governer (Towards the radiator ) it has a black cap . Unscrew the cap off and you will see a threaded stud that has a screw driver slot in the end . It also has a 7/16 lock nut. To increase air pressure loosen the lock nut . its been a while but Im almost certian its back the stud out to increase air pessure ( you will see after your adjustment), about 2 turns for about every 10 PSI of pressure. When its adjusted were you like , tighten down the locknut. There is a post you should read about the M920 that will help you alot. Its in the 5 ton section and covers alot. I'll see if I can find it.
 

Guam 7155

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Guam
I did all the tricks everybody and their mom suggested. Pourded oil in the umbilical cord and air tank, even removed the top cover plate on the valve body. Air leak got worse from the vent. Thinking the worse already.... (Rebuild). Anyone ever rebuilt one? Or any other suggestions.What's happening is, I'll let the air build up well over 120psi. Hit the shift button on the left side of the shifter. You then hear three clicking sounds, almost like something is engaging. (I think that's normal?). As soon as I hit the shifter to the left air starts coming out of the vent. What you guys think?
 

NDT

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Camp Wood/LC, TX
Hi Guam, welcome to the site. You will most likely not have to rebuild the transmission, only the valve body, which can be removed through the hole in the floor. You will need to search this site for posts made by "rracing" and the term "7155". He goes into great detail about what it took to overhaul the valve body. He was able to source the required O rings from Cat and Parker. The three clicking (clunking?) sounds is normal when you hit the button.
I was able to visit Guam about 10 years ago. I really enjoyed my stay. Is that Jap tank still sitting outside the machine shop? Good luck.
 

NDT

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Steel Soldiers Supporter
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Location
Camp Wood/LC, TX
Thanks NDT. Yup the tank is still there. Were you hear on vaction?
We were on our way to the Phillippines to buy US surplus, so it seemed like a great idea to stop off in Guam for some battlefield tourism. We met several MV collectors, the names of which I cannot remember.
 

OUR MURPHY

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PORT CHARLOTTE FL.
Hello i am "our murphy" i am new here mas im just registerred. I would like to say thank you to all members for their postings and ea special thank you to r racing you mhave been very helpful to me as i read your comments to many on the cat 7155. Everyone considers you the expert and it is easy to see why. I registered as our murphy because that is what my friends call our m 917 dump truck. It seems since we got it that what ever can go wrong will go wrong with it. But the longer we have it the more im starting to like it. Its a great truck and it sat a long time before we got it and when things sit idle they dry out or rust etc, our company was hoping it to be ready to go to work as soon as we got it and from a distance it looked ready to do so.
And we did. In the past year the longest it stayed running without repair down time has been about 3 weeks. But ol murphy is starting to shape up. Steelsoldiers has been a wealth of information so thank you all again. I thought it time i register so i can maybe give a little help back with some info to share .
 

NDT

Well-known member
Steel Soldiers Supporter
7,586
148
63
Location
Camp Wood/LC, TX
Welcome Our Murphy. Yup these trucks are 40 years old and lots of parts are at the end of shelf life. Once you debug all these systems you will have a reliable truck.
 
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